Melamine resins and urea resins constitute the most important representatives of the amino resins. They result from a kind of Mannich type reaction between NH-containing compounds, nucleophilic molecules and carbonyl-containing compounds. NH-Components are mainly urea H2N—CO—NH2 or melamine (2,4,6-triamino-1,3,5-triazine). The carbonyl component is predominantly formaldehyde (rarely ketones or other aldehydes). Nucleophilic components may be H-acidic (halogen acids), OH compounds (alcohols, carboxylic acids), or NH compounds (urea, melamine, amines etc.) The resulting “aminoplastics” are colourless. Melamine resins are used for shatter-proof tableware. Moreover, the literature reports the use of amino resins for the encapsulation of active substances and mentions in particular the potential use of such encapsulation systems in perfumery and cosmetic applications. Therefore, amino resin based capsules, also commonly designated as aminoplast capsules, are the subject of a variety of literature reports and patent applications relating to the perfumery and cosmetic industries. In practice, these polymers are capable of forming a protecting shell around the active ingredient that one wishes to protect, thus providing an encapsulation system characterized by its water-insolubility. The active ingredient protected by the capsule may be released through mechanical rupture of the microcapsules, which become brittle when dry.
The process for the preparation of aminoplast microcapsules containing encapsulated fragrances is a well-known state of the art and is described in the patent literature, for Example in U.S. Pat. Nos. 3,516,941, 4,406,816, 4,976,961, DE Patent 198,33,347 (BASF), WO Patent 01/51197 (BASF) and U.S. Pat. No. 6,261,483 (BASF), GB Patent application 2073132 and WO 98/28396.
In the state of the art, preferred melamine/formaldehyde microcapsules are prepared in the presence of anionic emulsifying polymers that control particle size distribution and dispersion of the resulting capsules during their preparation. Furthermore, the preferred anionic melamine/formaldehyde microcapsules are known to be highly impermeable to the encapsulated ingredients.
The use of such fragranced aminoplast capsules in liquid formulations for household and cosmetic applications is also well-known. For example, U.S. Pat. No. 5,188,754, assigned to Procter & Gamble, describes detergent compositions which contain perfumes in the form of friable microcapsules. U.S. Pat. No. 5,137,646, also to Procter & Gamble, describes the preparation and use of perfumed aminoplast particles which are stable in fluid compositions such as fabric softeners. However, this composition requires a two-step manufacturing process where the perfume is firstly solidified with a meltable polymer, followed by grinding of the solidified perfume and coating with the aminoplast resin.
It is also known that cationic transfer agents drive the deposition of such aminopast capsules on fabric, skin and hair. This is of particular importance when such aminoplast capsules are used in liquid rinse-off formulations like laundry detergents, fabric conditioners, shampoos, rinse-off hair conditioners and body washes.
U.S. Pat. No. 4,234,627, assigned to Procter & Gamble, discloses a liquid fragrance coated with an aminoplast shell further coated by a water insoluble meltable cationic coating in order to improve the deposition of capsules from fabric conditioners. In U.S. Pat. No. 4,973,422 (P&G), from 1989, it was then further described that capsules with a cationic coating provide improved substantivity to the surface being treated, such as fabric treated with a fabric softener. The same idea was described in 1991 in U.S. Pat. No. 5,185,155, assigned to Unilever, where the selection of cationic polymers was enlarged to water soluble polymers and the type of encapsulation was distinct from those in the state of the art at the time. Patent application US 20040071742, assigned to IFF, discloses a similar technology where the fragranced aminoplast capsules are coated with cationic starch or cationic guar.
However, such cationic coating procedures result in agglomeration of the capsules from the aqueous slurry. It is known in the art that the higher the cationic charge of the polymer, the faster the capsules will agglomerate. On the other hand a high cationic charge of the deposition polymer is desired to optimally drive deposition of the capsules during rinse applications. There is still a need to find a polymer combination that allows both cationic coating of the capsules and their homogeneous dispersion in the final aqueous slurry.
International patent application WO 03/002699, assigned to Colgate-Palmolive, describes fabric softening compositions where a cationic cross-linked polymer improves deposition of friable aminoplast microcapsules. In order to avoid agglomeration of the anionic capsules with cationic polymers, such cationic transfer agents are added to high dilutions of the fragranced aminoplast capsules in the final consumer base. The interaction of the capsules with the cationic deposition polymer is therefore limited and restricted by low concentrations of the capsules in the aqueous base. There is a need to optimize the deposition activity of such cationic polymers onto the capsule wall by increasing their direct contact with the aminoplast capsules in order to form a maximum cationic coating.
The improved deposition of cationic microcapsules in rinse-off formulations is also generally disclosed in US Patent application 2003/0171246, assigned to BASF, in US 2004007142 to IFF and in International patent application WO 01/62376, assigned to Henkel.
As previously mentioned, when cationic polymers are added to aqueous dispersions of aminoplast capsules, these capsules tend to agglomerate. In practical applications, the cationic polymers are added at low concentration to high dilutions of such aminoplast capsules. This reduces the formation of agglomerates liable to separate from the final rinse-off formulation.
However, the cationic coating of the aminoplast capsules is not very efficient and cannot be well controlled in order to maximize deposition of the capsules. Applying cationic coatings on aminoplast capsules through addition of cationic polymers to an aqueous dilution of such capsules will not lead to a uniform coating of these capsules and is therefore not efficient in increasing the protection of the encapsulated fragrance in such capsules against extraction by surfactants that are present in the rinse-off formulation.
Improved dispersion of melamine formaldehyde capsules in liquid formulations by adding dispersing agents is addressed in WO 03/089561 from P&G. Shear-thinning anionic and non-ionic polymers and silicas and bentonites are cited to prevent microcapsules from falling out of the solution. However, the invention does not address rinse-off formulations and the need to avoid agglomeration of anionic aminoplast microcapsules with cationic depositioning polymers.
As previously mentioned, the major down-side of the use of aminoplast capsules in rinse-off applications is that the perfumed capsules are not stable in such liquid formulations. The perfume gets extracted out/off the capsules by surface active ingredients in the formulation. So far, none of the aminoplast capsules described heretofore remain stable for 2 months at 45° C. in rinse-off formulations, i.e. under storage conditions that are encountered in many practical circumstances.
Some attempts have been made to increase the stability of such fragranced aminoplast capsules by modifying the capsules' membrane. For example, International patent application WO 02/074430, to Quest International, which outlines the above-mentioned stability problems of aminoplast capsules in aqueous surfactant-containing products, suggests a solution based on the use in the capsule shell of a second polymer comprising a polymer or copolymer of one or more anhydrides. It particularly describes improved stability in hair shampoo of aminoplast capsules prepared in the presence of ethylene(maleic anhydride) copolymer. While this solution improves the resistance of the capsules to degradation, it does not improve the deposition of such capsules during rinse-off applications.
Moreover, the resulting stability after 1 month at 37° C. is not sufficient for these applications.
On the other hand, WO 01/94001, assigned to Syngenta Ltd., mentions the possibility of having a solid permeable shell of a polymer resin having surface modifying compounds capable of reacting with isocyanate incorporated therein. While being based on the use of melamine resins, the latter shells have been modified so as to become permeable and, as a consequence, are susceptible of loosing the perfuming ingredients there-encapsulated through a diffusion process during the storage of the capsules in common consumer rinse-off formulations.
Therefore, the solutions provided until now by the prior art are not satisfactory.
The Applicant has also previously addressed this issue in patent application WO 2005/017085, wherein there is proposed a solution to the above-mentioned problem via the use of a special packaging system comprising two compartments, the fragranced aminoplast or coacervate capsules being lodged in a compartment which is separate from that which contains the surface-active ingredients commonly present in such rinse-off formulations. Although such a solution has proved to be a major improvement over previously described systems, because the fragranced aminoplast capsules have a lower density than other liquids which are generally used in liquid household and cosmetic formulations, the capsules may still tend to separate from the formulation. The same problem of separation applies when fragranced aminoplast capsules are added directly to liquid formulations where the density of the liquid is different from that of the encapsulated perfume. Such liquids include water, silicone oils, organic and mineral oils, alcohols, glycols and glycerine. It was observed that the lower the viscosity of the formulation is, the faster the capsules are likely to separate. Furthermore, the two-compartment packaging does not address the need to drive deposition of aminoplast capsules during rinse application onto the target surfaces.
In order to obtain a homogeneous dispersion of the capsules in a liquid formulation, this formulation needs to be thickened until it becomes a gel or a cream. Such thickened formulations are less convenient to dose and can no longer be easily sprayed.
We have now surprisingly discovered that by using specially selected combinations of water soluble or dispersible cationic and non-ionic polymers, dispersions of fragranced aminoplast or coacervate capsules can be stabilized in liquid formulations without reducing the flowability, spreading or sprayability of the dispersion.